Water gravity waves generated by a moving low pressure area

Hurricanes (typhoons) have an intense low pressure area which often moves rapidly over the surface of the ocean. The coupled water gravity waves generated by such a moving low pressure area have been studied in a towing tank. It was found that the waves generated in this manner had a phase velocity identical to the velocity of the moving low‐pressure area. When the low‐pressure area was moving at velocities less than the shallow‐water wave velocity , the waves were of nearly constant period. The period was that which when considered with the water depth would provide a wave phase velocity identical to velocity of the moving low‐pressure area. As the group velocity of water gravity waves is less than the phase velocity the wave groups were formed behind the moving disturbance. If the low‐pressure area is considered to be a gust it provides a mechanism for generating ‘wind waves’ of the period observed in the ocean if the forward speed of the gust is taken as the mean speed of the wind. When the velocity of the moving low‐pressure area approached the characteristics of the waves changed (at phase and group velocities become identical in the linear theory). The wave disturbance consisted of a large wave followed by a few smaller waves. After the pressure area movement was stopped, the first wave would move ahead of the tail of the disturbance, the distance between the two phenomena increasing with increasing distance traveled by the waves. For certain combinations of initial conditions several waves were formed with this characteristic. These waves had the appearance of the records on the tide gages at certain locations of the East Coast of the United States during Hurricane Carol (and other hurricanes). A study of Hurricane Carol with respect to its speed of forward movement and the water depth over which it moved showed that it should have generated waves of the type obtained in the laboratory and which were actually recorded.